1. The Field of the Invention
The present invention relates generally to the field of optical transceivers. More particularly, embodiments of the present invention relate to eye safety requirements for optical signals transmitted from optical transceivers.
2. Related Technology
Laser signals are widely employed in a variety of different technologies and applications. For example, lasers have been widely used in military contexts as range finders, as target designators, and in guidance systems. Lasers are also widely incorporated into communication systems for high-speed data transfer. The practical uses of lasers, as well as the physical properties of different lasers, vary greatly. While some lasers emit relatively low power signals, other lasers may emit signals of much higher power. In many instances, sensitive equipment, as well as human eyes, may be severely damaged by exposure to high power laser signals.
In order to protect human eyes from damaging laser signals, eye safety requirements have been developed to guide manufacturers of optical transmission devices. One set of eye safety requirements, Class 1 eye safety requirements, provide guidelines for safely transmitting laser signals in environments where unprotected eyes may be exposed to those laser signals. Class 1 eye safety limits incorporate limits on laser power and exposure time. Thus, the power of a laser signal may be high for a short period of time, or lower for a long period of time, and still conform to Class 1 eye safety requirements.
Class 1 eye safety requirements apply to the emission of laser signals in applications such as optical transceivers. For fiber optic transceivers, Class 1 eye safety requirements apply under all conditions, including all reasonable single fault conditions, which are defined as reasonable failures of a single component or connection within a transceiver. In order to comply with eye safety requirements, transceivers are generally designed to ensure eye safety in one of two ways. First, the transceiver may be fundamentally safe because the maximum power the transceiver can emit may be less that the eye safety limit. This is often the case with transceivers incorporating longer wavelength lasers that operate in the range of 1310–1550 nm. Second, for cases where the laser signal emitted from the transceiver may not be fundamentally safe, as, for example, with transceivers using lasers which transmit signals with wavelengths in the 850 nm range, the eye safety limit is ensured by redundant electrical circuits that monitor either the laser current, or, more directly, monitor the laser output power through a monitor photodiode.
While eye safety systems based on electrical circuitry are useful for keeping the power of laser signals within the eye safety limits, such eye safety systems may become complex and can increase cost, complicate production, and affect performance of optical transceivers. Eye safety systems based on electrical circuitry include redundancies to ensure that the optical transceiver will continue to function in the event of the failure of a single electrical component or connection within the electrical circuitry used in the transceiver. These electrical circuitry systems generally serve to cut off the bias current to the laser when a fault is detected, and therefore often consist of two transistors in series with the laser element. However, because the series components can reduce the electrical headroom within transceivers, thereby limiting transceiver performance, configuring transistors in series within transceivers may be impractical and inefficient.
Another example of redundant circuitry used to detect or compensate for single point failures involves the use of monitor photodiodes. In an eye safety system incorporating a monitor photodiode, the output of the monitor photodiode is monitored, and when the output exceeds a preset level, the laser bias current is restricted. In such an eye safety system, failure of either the monitor photodiode or the connection to the monitor photodiode must be detected because many systems use the monitor photodiode in a feedback loop to maintain the optical output power in a desired range. If the monitor photodiode, or the connection to the monitor photodiode, fails, the feedback loop will tend to drive the bias current to the maximum level, which in many systems would cause the output power level to exceed eye safety limits. Thus, a redundant system is needed to detect failures of the monitor photodiode, or the circuitry connection to the monitor photodiode, and shut down the laser independently. If all possible failure modes are to be addressed, the total eye safety circuitry can become complex, inefficient, and expensive. Moreover, it is possible to find reasonable single fault failures which will not be detected by a typical eye safety circuit.
Furthermore, design of shortwave optical transceivers is often complicated by the fact that the desired normal operating power is often quite close to the eye safety limit, thereby making design of a system to reliably distinguish between normal and unsafe levels challenging. In fact, the standards for acceptable output power are often defined by a minimum value and a maximum which corresponds to the eye safety limit. The desire to have the largest output power range for high manufacturing yield tends to make the problem of eye safety control more difficult.